A wound rotor motor (WRM) can be controlled using a voltage source inverter (VSI) in combination with a voltage source converter (VSC) so that variable motor speed and torque are obtained. An advantage of a this type of system is that only a fraction of the power delivered by the motor to the shaft which it drives must be handled by the drive system, i.e. the VSI/VSC combination. This is a desirable feature since the cost of the drive system is proportional to its capacity.
However, in order to operate under VSI control a WRM must first be started and brought from inactivity to some speed so that the rotor voltage does not exceed the voltage ratings of the drive system. In the prior art, this has been achieved by reliance on expensive starting resistors and contactors. FIG. 1 illustrates such a conventional system where power is supplied from a utility at 10 through a stator isolation breaker 20 connected to the stator of a motor 30. Some power flows out of the rotor of motor 30 during starting when torque is required at the motor shaft and voltage is blocked by inverter isolation breaker 40 from reaching the motor drive elements formed by conventional VSI 50 and conventional VSC 60. Instead, this power is diverted through starting breaker 70 to resistance means 80, which can, for example, be a liquid rheostat or other similarly functioning device, and is dissipated there in the form of heat. When the speed of motor 30 has accelerated to the point that the rotor voltage no longer exceeds the ratings of the VSI 50, breaker 70 opens, inverter isolation breaker 40 closes and the motor continues operation under the control of the VSI 50. The point at which the transition from resistance means 80 to VSI 50 operation occurs defines the minimum operating speed of the system under VSI 50 control.
The use of a low voltage VSI in a rotor circuit of a WRM provides for a robust and economical way to control torque in the motor in a limited speed range. This range is a function of the synchronous speed of the motor. Thus, in a typical application, the motor would operate under inverter control from 70% to 100% of its synchronous speed. In such systems, there must be a means to control the WRM during a start cycle such that it can be brought to the speed range within which the inverter controls operation. The components comprising such starter systems can be a very significant factor in the total cost of the system and therefore play a key role in determining its marketability.
What is needed is a more efficient and economical way to bring wound rotor motors to the desired speed range for inverter control.